Variable Speed Drives (VSD) for elevators are typically produced in two types, regenerative and non-regenerative. Both these types of VSDs are capable of producing alternating current (AC) or direct current (DC).
During each trip, an elevator either consumes energy or produces energy. When the elevator has an overhauling load, the motor functions as a generator and returns energy to the drive. A non-regenerative VSD will convert this energy to heat by use of dynamic braking (DB) transistors. A regenerative VSD will return energy to the AC power supply grid.
An elevator with a regenerative drive in total consumes no energy except for energy lost due to friction, power conversion, ventilation, illumination, signaling and control. An elevator converts kinetic energy to potential energy and then reconverts the potential energy to kinetic energy.
While regenerative power presents no problems for a power system, it can pose problems for emergency power systems. Often specifications require elevator drives that do not produce regenerative currents when the emergency power system is in operation. Virtually all high rise and most mid-rise buildings have emergency generators.
The conversion of energy from AC to DC and from DC to AC in elevator drive systems involves losses. In modern insulated gate bipolar transistor (IGBT) based VSDs, these losses are typically 3 to 5 percent per conversion. In a typical drive system, the drive receives three-phase AC power from a supply grid and immediately converts this power to DC. The DC power is applied to a DC bus that has a bank of capacitors connected to it. The capacitors' primary function is to eliminate any residual AC component from the DC power. This AC component is known as ripple.
An inverter is also connected to the DC bus. The inverter converts the DC to variable voltage variable frequency (VVVF) AC that is supplied to an AC motor. If the motor connected to the drive is a DC motor, as is often the case with modern elevators, then the inverter produces a variable voltage direct current (VVDC).
Emergency power generators normally are sized to run one elevator at a time. In an elevator system having more than one elevator, it is often the case that one elevator is producing energy while another elevator is consuming energy. The regenerated power has been used to operate more than one elevator using a generator that would otherwise run only one elevator at a time. However, the power transfer in the prior art takes place via the AC power bus. Transfer on an AC power bus requires two additional AC/DC conversions at a loss of 6 to 10 percent.